When hunting animals, especially with bows and arrows, spears, harpoons and other shafted ordnance, it is often necessary to track an animal that has been shot over many hours. Tracking an animal through the use of a radio transmitter attached to the animal can be superior to other methods but has its own difficulties. If the antenna is external to the animal, the antenna can often be rubbed off on a tree, rendered non-functional by the animal falling down on top of the antenna, or by other movements of the animal. Such problems attend embodiments of U.S. Pat. No. 7,300,367 to Andol et al., for example, which teaches a tracking assembly having hooked barbs that enter the hide/skin of the animal, leaving a transmission module attached to the outside of the animal.
If the antenna is internal to the animal, then the signal is often so attenuated by the body that the signal is too weak to track. One cannot merely increase the signal power because (a) the drain on the battery would likely be too great, and (b) the degree of signal attenuation would vary so much depending upon placement of the shot, that some shot placements would result in an illegally strong signal. Such problems attend embodiments of U.S. Pat. No. 4,976,442 to Treadway et al., which teaches an arrow shaft mounted transmission module that is released from the shaft as the shaft enters the animal. Similar problems attend shots placed with a bullet rather than a shafted ordnance, both against hunted animals and in military or police operations where the ordnance is used against a human. As used herein, the term “animal” should be read to include a human.
Andol and Treadway, as well as any other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Looking to implanted medical devices for solutions provides many examples of small modules with integrated antennas that are placed within a body for diagnostic purposes. These implanted modules use a low power data transmission to an external monitoring device typically using the Medical Implant Communication Service (MICS) bands (401 MHz to 406 MHZ) or the Wireless Medical Telemetry bands (608 MHz to 614 MHz, 1395 MHz to 1400 MHz and 1427 MHz to 1432 MHz). Other frequencies commonly used are the 915 MHz and 2.45 GHz bands in the Industrial, Scientific and Medical Equipment (ISM) bands. It turns out, however, that such frequencies are not suitable for tracking animals because the transmissions are attenuated much too quickly in the body and over long distances at the low power needed to meet regulatory requirements.
The main reason for using a higher frequency is that the size of the antenna is smaller than for a lower frequency, since the length of the antenna is dependent on the wavelength of the RF signal. In general terms, as the frequency of an RF signal increases as the wavelength of that RF signal decreases.
More specifically, the wavelength of an RF signal is dependent on the frequency and the dielectric constant of the material through which the signal is travelling. Since the dielectric constant of dry air is relatively constant over frequencies up to 100 GHz, the wavelength of an RF signal in dry air is also relatively constant and easy to calculate. In contrast, the various tissues in a body each have different dielectric constants that are frequency dependent, so the wavelength of an RF signal travelling through a body varies with the different tissues it passes through and with the frequency of the signal, thus calculating the wavelength of an RF signal in a body requires knowing the dielectric constants of the various body tissues at the frequency of the RF signal and is usually reduced to an approximation using a composite equivalent dielectric constant for a typical body.
Another aspect of the frequency dependency of the dielectric constants of various body tissues is that the absorption of an RF signal by the body increases with frequency. For frequencies under about 4 MHz, the wavelength of the signal is significantly larger than the cross-section of a typical human body and there is very little effect on the signal. Above 4 MHz, the absorption of signal energy increases in proportion to the increase in frequency until the human body becomes essentially opaque to RF signals. And above about 1 GHz, the different dielectric properties of the various body tissues begin to cause diffraction and refraction of the RF signal at the tissue boundaries.
The frequency dependency of the dielectric constants of various body tissues also affects the efficiency of implanted antennas for medical applications, which typically only achieve an efficiency of 0.01% to 3% as compared to antennas out in the open air that can usually achieve 95% efficiency.
The higher the frequency, the higher the absorption by the body and the resulting loss of RF signal strength as it passes through a body. As a result, it is desirable to keep the frequency of transmissions for a tracking device as low as practical to reduce the attenuation of the RF signal by the body to which the tracking module is attached. The lower limit for a practical frequency is determined by the length of the antenna needed.
Since the conductivity differential between blood and other body tissues is typically at least 5:1, coupling an RF signal to the blood in the circulatory system, using a matching network to maximize signal transfer, will essentially use the blood as the conductor of a large area, lossy fractal antenna. The effective length of a circulatory system antenna would be dependent on the placement of the wireless device in the body relative to the extremities, but would be several orders of magnitude longer than any antenna that could be contained in (or on) the wireless device.
And while the transmission losses would be rather high, depending on the RF signal frequency and the dielectric constants of the surrounding tissues, the overall RF transmission efficiency should be at least as good as for antennas currently in use with implanted wireless devices.
Thus, there is still a need for an ordnance usable against a human or other animal, which provides a good tracking signal where the antenna is internal to the body.